1. Field of the Invention
The present invention relates to power supplies in general and more particularly to power supplies for energizing stepper motors.
2. Prior Art
The use of stepper motors to perform a variety of industrial tasks is on the increase. The typical stepper motor includes a plurality of independent windings which must be controlled in a predetermined sequence in order to produce an incremental step. The control signals are generated from a control means which accepts a series of command pulses and provides control signals for controlling the windings. The control signals force the motor to step a predetermined distance. The step is, essentially, produced simultaneously with the appearance of the pulse. In addition to the stepping pulses, each winding must be supplied with energizing current. The energizing current is usually provided from an adequate power supply.
Designing adequate power supplies has been a problem in the past. The problem stems from the fact that low voltage stepper motors require a relatively high starting torque and a relatively low running torque. Motor torque is directly related to the current flow in the motor windings. If the current flow is high, the torque is high and vice versa. One of the inherent characteristics of a low voltage stepper motor is that the coil inductance is relatively high which in turn limits the rise time of coil current. Since the natural characteristic of the coils, in the low voltage motor, limits fast current rise and the desired result is to maximize current flow in order to provide the high startup torque, the designer faces a dilemma of reconciling these conflicting requirements.
The problem is further augmented because in several applications the motor must respond within a relatively short time interval. However, friction and other motion opposing forces are initially large and a high startup torque is necessary to force the mechanical load into motion. Once started, the inertia of the load will reduce the motor torque requirement.
The prior art methods for solving the low startup torque problem can be broadly classified into two main circuit types, namely: chopper driver circuits and bilevel or pedestal driver circuits. Although both of these approaches work well for the intended purpose, they suffer from a common drawback, namely: a much higher voltage than the rated motor voltage is used to energize the motor. The high voltage causes the coil current to rise quickly thus increasing the startup torque.
Regarding the chopper drive circuit, it is designed to pulse a high voltage across the motor coil at a relatively high frequency (say, 20 kHz). During the pulse, the coil current rises rapidly and decreases slowly when the pulse ends.
As a result, a high startup torque is obtained with a relatively low DC current. In addition to the above-identified problem, these circuits suffer from high component count, noise at the switching frequencies and low efficiency due to switching losses in the motor and the active devices which form the circuit.
On the other hand, the pedestal or bilevel driver circuits apply a high voltage across the motor coil until the current reaches a predefined level whereupon the high voltage power supply is switched out of the circuit and a lower rated voltage supply is used to drive the coil. The major disadvantages with this type of circuit are the requirement for at least two power supplies, noise isolation between the supplies, protection required when the power supplies are switched and low efficiency.
The above-described circuits and other types of circuits aimed at solving the high startup torque problems are given in the following prior art patents: U.S. Pat. No. 4,295,083; 3,967,179; 3,809,991; 4,353,021; 4,208,868; and U.K. Pat. No. 1,579,121.